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RE: Equalization


I was skeptical, like you, about the ability of equalization to
overcome DMD. I still am. But what made me more open minded is my
willingness to examine two key propositions: DMD behavior can be
approximated as that of a fading multipath channel. And equalization
can overcome the effect of a fading multipath channel.

Why fading multipath? Consider this. A multimode fiber with DMD is
seen by an optical signal as a channel that propagates various
portions of its energy (modes) through a refractive index profile
that is sharply different (at places) than intended. The propagation
velocity of a mode depends on refractive index. Over distance, on
average, some modes will have a cumulative average of a low (lower
than intended) refractive index path, thereby abnormally decreasing
their path delay, while others may not. In some bad cases, a bit
arriving at the receiver is almost split in two or three replicas.
And the amplitude of each replica is changing dynamically. Guess
what, that is not far from the behavior of a fading multipath
channel. Equalizers that deal with this phenomenon are used in TV
de-ghosting circuits and digital radio.

To an equalization expert, DMD may not look so challenging - it's
just another channel with randomly time-variant impulse

But then, we both are putting the cart before the horse. Let's wait
to hear some presentations from equalization experts. Even if it can
be done, I will want to know whether it can be done cost-effectively
and on time. Our discussion should serve as a guide to them about
what we would like to hear.